(i) Field of the Invention
This invention relates to a drill screw.
(ii) Description of the Prior Art
A drill screw comprises an elongated shank having integrally formed thread convolutions of a substantially uniform outer diameter and a substantially uniform root diameter; a head is disposed at one end of the shank and is provided with means for interengaging a tool to turn or drive the screw into a workpiece and an entering end portion or drill tip is disposed at the other end of the shank.
This type of drill screw has the function of opening a hole in a workpiece, cutting female threads in the hole and screwing the workpiece in place in a single operation.
Thus no separate operations of boring or countersinking are employed when using a drill screw; and the drill screw forms the hole and screws itself into place in a single operation.
When this type of drill screw is used, it is usual practice to screw a workpiece in place by using a large number of drill screws.
It is thus desirable to employ drill screws which can be driven into the workpiece rapidly so that the operation can be completed in a short period of time.
A particular problem faced by users of drill screws is the hole forming ability of the drill screw, and this affects the overall operation efficiency; for example, chips of metal formed in the cutting or hole forming tend to be trapped between the drill screw and the partially formed hole. Such chips may cause jamming such as will prevent further hole opening operation of the drill screw.
If the drill screw can not be freed from the jamming chips it is left in a partially screwed in position and a new attempt made to drill in a drill screw in an adjacent location.
The jamming can sometimes be overcome by increased thrust applied to the drill screw, however this makes the drilling operation more arduous, and the person drilling becomes fatigued in a shorter period. Another problem is that application of a larger thrust may overload the motor of the drill screw appliance.
The thrust which is required to cause a drill screw to penetrate a workpiece, the time required for the drill screw to effect such penetration and the efficiency with which the drill screw can fasten the workpiece in place are all governed by the ability of the drill screw to form the hole.
If the hole forming ability of a drill screw is low, then a thrust of a larger magnitude and for a longer period of time will be required, than are normally required to cause the drill screw to penetrate the workpiece. In this event, difficulties are encountered in fixing the workpiece in place and consequently overall efficiency is lowered.
Known drill screws also have the drawback that the flutes are choked with chips produced in drilling a workpiece of aluminium and further drilling becomes impossible.
In a known drill screw described by reference to FIG. 6 of U.S. Pat. No. 4,147,088, Whittaker (presented as FIG. 3 in this specification), chips flowing on the cutting surface 52 in the direction shown by the arrow in FIG. 3 run against the drag surface 54 once and are discharged thereafter. In view of the resistance and the relatively small volume of the flutes the chips are not efficiently discharged.
U.S. Pat. No. 3,786,713, Sygnator, describes a drill screw shown in FIG. 4, in which the flutes of the screw are provided with protrusions for breaking chips into small pieces. These protrusions provide a high resistance to flowing chips and the chips thus cause jamming, which interferes with the efficient operation.
Generally, the entering end portion or drill tip of the drill screw comprises a cylindrical portion coaxial with the shank of the drill screw, two flute surfaces formed in the cylindrical portion in such a manner that they are inclined with respect to the center axis of the cylindrical portion, the flute surfaces being disposed on opposite sides of such center axis, and the cylindrical portion having end surfaces in the form of a roof.
The entering end portion or drill tip of a drill screw may be formed by two processes: machining and forging or pinch pointing. Drill screws produced by machining are superb in drilling efficiency but high in cost. Drill screws produced by forging or pinch pointing can be produced at low cost but are inferior in drilling efficiency as compared with machined drill screws.